只接受非负数Java

Question

else if (bb.equals("Dollar") && cc.equals("Pound")){
        input = JOptionPane.showInputDialog("What amount would you like to convert?:");
        double aa = Double.parseDouble(input);
        double output =(aa*rate);
        JOptionPane.showMessageDialog(null, String.format("Your amount is: $%.2f",  output));

如何做到这一点，如果有人输入负数，会弹出一条消息说无效输入？

Answer 1

我假设您要检查的数字是否由变量aa定义为正数？我个人更喜欢在处理货币价值时使用BigDecimal，在这种情况下我会这样做：

BigDecimal aa = null;
try {
    aa = new BigDecimal(input);
} catch (NumberFormatException nfe) {
    //Not a numerical value, throw an error
}
if (aa.compareTo(BigDecimal.ZERO) <= 0) {
    //Value is negative
}